This invention relates to elevator systems and specifically to methods and apparatus for fabricating and installing thermoset plastic inserts in the metal drive sheaves frequently used in such systems.
In the typical elevator system, multistrand steel ropes or cables ride on large metal drive sheaves that are rotated by an electric motor to raise and lower the car. The car is attached to a counterweight by the ropes and the car and counterweight hang from opposite sides of the sheaves. Thus, the counterweight balances against the weight of the car, and reduces the lifting force. The counterweight and car move in opposite directions in the shaft: as the car goes up the counterweight goes down and vice versa. The sheaves thus support the combined weight of the car, the load and the counterweight, or roughly twice the loaded car weight.
The ropes typically rest in slots in the rim of the sheave. The depth of these slots, in many cases, is greater than the diameter of the rope; the rope thus does not rest on the bottom of the slot, and, as a result, it is squeezed between the walls of the slot by the downward pull from the car and counterweight. This aids in providing good traction between the sheave and the rope to minimize rope slippage as the sheaves are rotated.
Sometimes the thin wire strands that make up the ropes are woven in a direction at right angles normal to the direction of rope and sheave movement to improve traction; this is commonly known as a Lang lay.
Consequently, as the rope passes around a rotating sheave, it is repeatedly squeezed and released as it passes in and out of the slot. This may produce accelerated rope wear, especially along the rope edge as it repeatedly rubs or scrapes on the slot walls. It may also cause the rope to flatten slightly (to change from its normal generally round shape to the shape of the slot). These adverse effects are particularly prevalent in insertless sheaves, and may produce unacceptable rope slippage on the sheave, especially as the slot wears. In a typical installation the only way to correct this is by complete sheave and/or rope replacement, which can be expensive and time consuming, especially in large buildings, where the ropes are very long. Thus, minimizing rope wear translates directly into increased service life, reduced operating costs and increased reliability.
A successful technique utilized in the prior art to minimize the occurrence of these adverse effects, and also at the same time reduce noise and vibration transmission and increase rope traction at the same time, is to place a thermoset plastic insert into the lower portion of the slot. The rope rests on the insert, which provides a high traction, soft, load bearing surface that prevents the rope from being pulled into the lower portion of the slot, where it may be undesirably squeezed or compressed. Such inserts are disclosed in U.S. Pat. No. 3,279,762, which is commonly owned herewith. Such inserts can significantly extend rope life, and, of equal importance the traction relationship between the sheave and the rope is not determined, to any significant degree, by the extent to which the rope is squeezed between the slot walls. Inserts therefore serve to reduce the otherwise critical relationship between rope size and slot dimensions for good traction. To enhance these features, inserts frequently have traction cleats oriented normal to the direction of shaft rotation, as disclosed in the aforementioned patent.
The inserts typically are constructed of molded thermoset plastic, such as polyurethane, which is particularly desirable as it has high durablity and inherent elasticity. The inserts may be installed in short segments around the sheave, or preferably as a continuous, closed loop, which may be installed like a tire is on a rim: by stretching it around the outer edge of the sheave and working it into the slot. Because it has inherent elasticity, a polyurethane insert loop will be pulled or drawn into the slot if its overall circumference is slightly less than the circumference of the sheave, as measured in the groove into which the insert is placed; thus the insert is desirably tensioned (stretched) in the slot, preventing its movement therein.
A thermoset insert, i.e. one constructed of polyurethane, with cleats, is fabricated by molding liquid polyurethane material and curing it under temperature. Such an insert may be joined into a homogeneous loop of insert by joining the ends together; this may be accomplished by placing additional liquid polyurethane between the ends, holding them in place while the liquid cures and applying heat to accelerate curing. The maximum temperature must be controlled, however, because excessive heat can cause damage to the insert material. Moreover once a thermoset plastic material has cured, it generally may not be remolded simply by heating. In some instances heating may even impair the material's flexibility and durability. Other plastics (i.e. styrene) are not as adversely effected by heat: they can be remolded. But their durability, especially under frictional loading, is not nearly as good as thermoset plastics. Consequently, laminating or adhesively bonding the ends of thermoset inserts (i.e. polyurethane) must be done with sufficiently good heat control.
In many multisheave installations, the sheaves are mounted on a single shaft supported at each of its ends by a large bearing stand that is bolted to the floor of the elevator machine room. These bearings sustain the aggregate weight of the elevator car and counterweight. Installation of "looped" inserts in such arrangements requires disassembly of at least one bearing stand so that the loop may be passed around the shaft and worked into place on the sheave. This applies to both insert replacements and retrofits for sheaves without any inserts.
Consequently, installation of a homogeneous or looped insert may be expensive and time consuming. The need to block the car and completely slacken all the ropes, when this is done, further adds to the cost, and even if only one insert is to be installed, that procedure must be followed, since the stand has to be completely disassembled on one end; that can only be done by releaving the load on every sheave. In contrast, if the stand does not have to be disassembled, only the cable on the sheave being serviced has to be slackened. Since heretofore the replacement loops could be made only in a factory, complete disassembly of the stand was required.